This invention relates to the completion of lateral well bores, and in the preferred embodiment provides a relatively simple and inexpensive arrangement which facilitates passage of relatively large diameter objects into a completed lateral well bore whilst permitting passage of relatively large diameter objects past the lateral well bore into the portions of the original well bore located below the lateral.
In recent years, the technique of producing lateral well bores (also known as “laterals” and “branch well bores”) has been significantly developed. Laterals are well bores which are drilled to branch off an existing main well bore in order to gain access to the strata surrounding the main well bore. Typically, one or more laterals will be drilled from a main well bore starting from a point somewhat above hydrocarbon bearing strata surrounding the main well bore. The laterals are generally bored away from the main well bore in a generally downward direction (although horizontal or even upwardly extending laterals are known) to arrive in the hydrocarbon bearing strata at a point displaced from the main well bore. By this means, hydrocarbon material can be extracted from the formation surrounding the main well bore without drilling a fresh well bore from the surface.
In order to maximise the benefit of the lateral technique it is common to bore several laterals spaced apart angularly around the main well bore. Such arrangements are known as “multilaterals”. A number of arrangements for drilling laterals and multilaterals are disclosed, for example, in published International patent application WO 94/03699.
Especially in the case of multilaterals (although to an extent also in the case of single laterals) it is often desirable to have communication from a point in the main well bore above a lateral to a point in the main well bore below the lateral. Such communication may be used, for example, to permit hydrocarbon material to flow from a lower lateral past an upper lateral to the surface. In more sophisticated arrangements, access may be required past a lateral to allow service or development tools to be run past the lateral to work at a location below the lateral.
A number of arrangements have been proposed to facilitate this desirable communication between portions of the main well bore above a lateral and portions of the main well bore below the lateral.
In one early example of these techniques disclosed in U.S. Pat. No. 245,920 (Shell) a continuous casing is inserted to extend from a point in the main well bore above a lateral to a point below the lateral. After cementing, the main casing is perforated to provide fluid communication between those portions of the original main well bore located below the lateral and those portions of the original main well bore located above the lateral.
In WO 94/03699 a number of techniques are disclosed. In particular, in one technique it is proposed to install a lateral liner so that a portion of the liner resides in the main well bore, to cement the lateral liner in position, and then to wash over the portions of the lateral liner which reside in the main well bore with a view to producing a clean inverted Y junction. Also disclosed in WO 94/03699 is a technique whereby the lateral liner is installed with a portion of the lateral liner located in the main well bore above the lateral junction, the lateral liner is cemented in position, and a milling tool is run through the lateral liner along the central axis of the original main bore to form a milled opening in the lateral liner to facilitate communication between the portions of the main bore located below the lateral and portions of the main well bore located above the lateral.
More recently, U.S. Pat. No. 5,477,925 proposes use of a pre-formed window element which forms part of a completion string for the lateral. As proposed, the window element includes means for hooking onto the window formed in the casing of the main well bore. Such an arrangement ensures that the window element is located at precisely the correct depth relative to the window in the main well bore casing so as provide through communication between the portions of the main well bore located respectively above and below the lateral. Whilst this device represents a substantial improvement over the techniques of earlier proposals, the system does suffer from the advantage that the entire completion string must be correctly angularly oriented throughout the process of running the completion string into the lateral. This is because it is only by maintaining the correct angular orientation of the completion string that the window element will be presented to the main well bore casing window at the correct orientation to ensure seating.
All the above arrangements are characterised by one or more disadvantages. The technique of U.S. Pat. No. 2,452,920 provides fluid communication but does not permit the passage of tools between the portions of the main well bore above and below the lateral. Also, the technique of U.S. Pat. No. 2,452,920 requires precise positioning of a perforating tool which may not, in practice, be possible in many cases. The washover technique of WO 94/03699 can produce a clean inverted Y junction. However, the technique requires an additional somewhat uncertain operation (the washover operation) and can result in a unstable junction if applied to laterals drilled into unstable formations. The mill through technique of WO 94/03699 obviates the problem of an unstable junction but may be difficult to achieve in practice because the mill tool will tend to follow the lateral liner into the lateral rather than bore straight through the lateral liner as suggested by the illustrations of the patent specification. The technique of U.S. Pat. No. 5,477,925 requires the maintenance of the correct angular position of the lateral lining as it is run into the lateral to ensure that the window element is at the correct angular position relative to the main well bore casing window.
We have now devised a relatively simple technique which provides communication between the portions of a main well bore located above and below a lateral and which overcomes the disadvantages of the prior art outlined above.